Precipitation can be defined as a physical-chemical process, in which soluble metals and inorganics are converted to relatively insoluble solids known as precipitates. The physical process of precipitation involves adjustment of concentration and or temperature of the solution to an extent where crystallization process takes place. Under physical precipitation, no chemical reagents are added to the solution. Physical precipitation is a slow process and usually the crystal products obtained have high solubility in water and contain water of crystallization. Chemical precipitation on the other hand, involves the addition of reagents to precipitate a desired compound from solution (Habashi, 1999). This work focused on the precipitation of metallic ions through addition of chemical agents.
2.4.1.
Precipitation of metals as sulphides
Metal sulphide precipitation can be applied in the separation of impurities from aqueous solution or in the recovery of major metals of interest from solutions. In metal sulphide precipitation process, soluble metal compounds are converted into relatively insoluble sulphide compounds by addition of agents containing sulphurous atom. Such sulphide precipitating agents may be in gaseous (H2S, SO2), aqueous (Na2S, SO32-, SO42-) and or solid (CaS, FeS) form. Equation 24 illustrates the reaction mechanism involved when a sulphide ion is used in metal sulphide precipitation.
M2++ S2-→ MS(s) Equation 24
where M2+ is the metal cation being removed from the solution.
For the metal ions pertinent to this research the reaction would occur as given below:
Cu2++ S2-→ CuS(s) Equation 25 Ni2++ S2-→ NiS(s) Equation 26
2Rh3++3S2-→Rh2S3(s) Equation 27
2Ru3++3S2-→Ru2S3(s) Equation 28
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The use of metal sulphide precipitation technology is preferred in hydrometallurgical treatment of metal containing aqueous solution to hydroxide precipitation. This is so because metal sulphide precipitates produced have lower solubilities, favourable dewatering characteristics, potential for selective metal removal, and high degree of metal removal at relatively low pH (Bhatthacharyya et. el, 1981, Lewis, 2006, Peters &Ku, 1985). The general solubilities of common metal sulphide in order of most soluble to least soluble are given by: FeS >ZnS >NiS >CoS >PbS >CdS >CuS >Ag2S >HgS >Ir2S3 >Rh2S3 >PtS2>RuS2 >OsS2 >Au2S3 (Thomas, 1964). In addition, sulphide precipitation process requires relatively low detention time in the precipitating reactors because of the high reaction rates of sulphides. Furthermore, sulphide precipitation is less affected by the presence of complexing and chelating agents than hydroxide precipitation (Crear, 2001). The application of sulphide precipitation method in the removal selenium and tellurium from process solution at Lonmin BMR has already been discussed in section 2.3.
2.4.2.
Key results from previous OPM chemical precipitation investigations
There are several chemical precipitating processes employed in the recovery of metallic compounds from leach solutions and other aqueous solutions. This section discusses several researches done on the application of chemical precipitation process to recover metals from acidic process solutions using various reducing agents.
Awadalla et al. (1994) investigated the direct recovery of PGMs from thio-urea solutions and other highly acidic leach solutions by reduction precipitation using sodium borohydride at
ambient temperature and pressure. Solutions containing 5 ppm – 1000 ppm PGMs and 25 ppm – 250 ppm Cu2+, Pb2+, Zn2+, and Al3+ were investigated at temperatures from 25oC to
60oC over a time range of 2 – 15 minutes. The method was observed to be effective over a wide
range of solution acidity (from pH <1 to 4). Approximately 95 % maximum PGM recovery was achieved in their study. They also observed that reduction of PGMs was more efficient at lower
PGM concentration not less than 25 ppm. They further observed that the presence of Pb2+, Zn2+,
and Al3+ within the given concentration range had no effect on the PGM precipitation efficiency
and kinetics. The Pb2+ was, however, observed to co-precipitate with the PGMs. The presence of
Cu2+ ions in the solution has a negative effect on the use of NaHB4 to precipitate PGMs. This is
because copper ions acts as an active catalyst for the hydrolysis of borohydride ion and rapidly
liberate hydrogen before the PGMs are completely reduced. A Cu2+ concentration above 25 ppm
was observed to have a large effect on PGMs precipitation with Rh being the most affected. It
should thus be noted that using NaBH4 to precipitate PGMs in the current study would produce
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CuSO4 process solution under investigation. Furthermore, using sodium borohydride would
introduce sodium into the solution which is regarded as an impurity in Ni recovery.
Many other reducing agents that are capable of precipitating metals from aqueous solutions
have been discussed. McGeorge et al. (2009) used Na2S2O3 as a source of sulphur atoms to study
the mechanism and kinetics of Rh and Cu co-precipitate from aqueous solutions. A synthetic solution containing 13.2 g/L Cu, 5.5 g/L Ni and 90 mg/L of Rh was used for their investigate
over a temperature range of 50 – 150oC. In the first phase of their experiment, Rh precipitation
in the absence of copper was examined followed by Rh precipitation in the presence of copper. They found that in the absence of Cu2+, ionic Rh precipitation (equation 31) took place. This reaction is fast and it is where most of the Rh is precipitated due to the availability of sulphide ions provided by the precipitating reagent. When copper is present in the solution, ionic co- precipitation occurs. These reactions occurs homogeneously (equations 30 and 31) preceding noticeable nucleation which is followed by heterogeneous crystal growth (equation 32) before Rh co-precipitate with copper (reactions 33 and 34) (McGeorge et al., 2009).
Homogeneous reactions:
Cu2+ + S2O2-3 + H2O →CuS + H2SO4 Equation 30 2Rh3+ + 3S2O2-3 + 3H2O→ Rh2S3 + H2SO4 Equation 31 Heterogeneous growth:
Cu2+ + (CuS) x. nH2S → (CuS) (x + 1). (n – 1)H2S + 2H+ Equation 32
Co – precipitation
Rh3+ + (CuS) x. nH2S → CuRhS2. (n – 1)H2S + 2H+ + e- Equation 33 2Rh3+ + (CuS)x. nH2S → CuRh2S4. (n – 3)H2S + 6H+ Equation 34
The initial reactions are characterized by high consumption of the reagent since copper initially precipitate faster than Rh because of the large concentration difference between the two
components. Large concentration amount of Cu tends to limit the amount Rh3+ ionic
precipitation by consuming the available sulphide. In such cases the CuS formed continue to precipitate Rh3+ via cationic substitution reaction resulting in the enrichment of the Rh3+ towards the edge of the CuS particles (Mc George et al., 2009).